So the real question comes down to this: why isn't there a Perl CMS platform like Drupal?

Because nobody has written a Content Management System like Drupal for Perl yet, possibly because Drupal already exists, and it's more work to re-write it in Perl than to learn enough PHP to tinker with it. I know that's not great for Perl enthusiasts. However, to say that Perl has no content management systems isn't correct either:

Those are just a couple that I know of. I imagine that there's room for improvement with both of those. I know that Galileo is under active development. I wouldn't be surprised if collaboration is encouraged.